Generally, a pressure diffuser useful in the paper pulp industry comprises an elongated generally vertically upstanding vessel which mounts an elongated annular screen for vertical movement within the vessel. Pulp under pressure flows into one end of the vessel and into the annular space between the screen and the exterior vessel wall and through an outlet adjacent the other end of the vessel. As the pressurized pulp traverses the height of the vessel, displacement liquid is introduced into the annular chamber by a plurality of vertically spaced header assemblies. The displacement liquid flows generally radially inwardly through the pulp, treating the pulp, and through the screen into the interior of the vessel furnished with a liquid outlet. The screen is moved concurrently with the pulp a limited distance of travel and is then returned quickly to clean the screen by a combined wiping and backflushing action. The different diameters of the upper and lower ends of the screen create filtrate compression and thus backflushing (e.g. see U.S. Pat. No. 4,396,509), during screen movement thus forcing the liquid through the screen holes to backflush the screen.
While such pressure diffusers have been used successfully, a drawback of this type of flushing action is the pressure shock generated in the pulp by the backflushing action. For example, the quick motion of the screen tends to suddenly compress the pulp adjacent the pulp inlet. This has a deleterious effect on the process.